For multihead drives, only one head can be placed on the carriage with the trajectory of the center of the objective lens constantly passing through the center of the disc when the carriage is traveling in the radial direction while accessing the same region of the disc. Relative to that head, any other head, if located on the same carriage, will have to be mounted with some offset. Therefore, positioning of these two heads normal to the same track becomes possible only at one radius of the disc. With a radially moving carriage, at any other radius each of the two heads will be reaching different tracks, and at most one head will be kept normal to the track. If, for instance, such a two-headed arrangement is used in a Direct Reading After Writing (DRAW) drive, direct reading after writing cannot be executed.
A similar two head arrangement is also used in some Digital Versatile Disc (DVD) optical drives where one head reads DVD discs while the required reading of CD discs is executed by the second head. The latter is located at some distance from the center of the carriage (which passes through the center of the disc) that changes the angular orientation of this head with respect to different tracks when the carriage is moving radially from the inner to the outer radii of the disc. Although such angular errors in reading CD discs can be somewhat compensated, the overall radial travel of the carriage must be increased to permit the CD head to reach all tracks of the disc. As a result, the carriage guiding elements together with the linear motor to drive the carriage must be elongated that represents a certain disadvantage of these drives.
In a separate case, as described in EP 727776-A1, information from different recording media is read by one head but equipped with two objective lenses. The latter are mounted on a turning plate included in the pin type fine focus/tracking actuator to permit each lens to be separately brought into the light path to read one or another recording media. The disadvantage of this design may be attributed to the fact that the actuator becomes substantially larger. The increase in moving mass also makes the lens driving motors less efficient. Additionally, since a large turning angle is required to bring each lens into its working position, the air gaps in each of the two focus motors become unbalanced resulting in a combined focus force displaced from the center of the active lens. As a consequence, dynamic behavior of the actuator becomes more complex.
In optical data recording, an optical source, typically a laser or laser diode, generates an incident write signal in the form of a radiation beam. The beam is applied to an optical medium to record data thereon as optically-detectable marks. The quality of recorded data in many optical recording systems is generally very sensitive to laser write power.
In most recording systems, however, optical recording power (ORP) may need to be adjusted during recording because this power varies with fluctuation of different parameters such as media sensitivity, defocus, tilt, substrate thickness, birefringence, scratches, and contamination on the laser-incident substrate surface. One technique for continuously maintaining the ORP involves monitoring a reflection of the write signal from the medium, known as the Mark Formation (MF) signal, while data is being recorded. Systems which monitor the MF signal are generally referred to as Direct Read During Write (DRDW) systems. The MF signal is also used to avoid the need to subsequently read the data after recording by analyzing the MF signal to determine whether or not a mark has been properly formed, or, in other words, whether the data has been properly recorded on the medium.
A technique to servo the laser write power on optical recording media during recording is disclosed, for example, in commonly assigned U.S. Pat. Nos. 5,436,880 and 5,446,716, the disclosures of which are incorporated herein by reference.
Apparatus for generating a mark formation effectiveness signal (MFE) to verify data as it is recorded on an optical medium is disclosed in commonly assigned U.S. Pat. No. 5,495,466, the disclosure of which is incorporated by reference. As disclosed in this patent, the MFE signal estimates a normalized rate of change of the reflected write pulse as a mark is being formed to provide an indication of the quality of mark formation on the optical medium. A technique to generate an MFE signal to control laser write power during recording is also disclosed in U.S. Pat. No. 5,216,660.
These DRDW systems suffer from the difficulty of monitoring mark formation during the write pulse due to considerations of dynamic range and write laser noise. Also, mark formation usually occurs during the cooling process after the thermal recording. The use of a trailing read spot for direct read after write (DRAW) offers a much higher quality mark formation monitoring. For a more complete discussion of DRAW, see Chapter 11 of Optical Recording, by Alan B. Marchant, Addison-Wesley Publishing Company, New York, 1990.